1. Field of the Invention
The present invention relates to imaging systems. More particularly, this invention relates to a dynamic focusing apparatus for increasing the depth of field of an imaging system, such as an optical scanner.
2. Description of Related Art
Imaging systems are currently used in a wide variety of applications including materials handling, materials inspection, package routing, data acquisition, document tracking and inventory control. Many of these applications require accurate, high speed imaging of a particular region of interest and acquisition of data pertaining to objects within that region. Due to their accuracy and versatility, imaging systems have been increasingly used for detection and decoding of human or machine readable symbologies. In particular, bar code symbols located on items have been successfully used to increase the speed and accuracy of the tracking of items.
Imaging systems include optical laser scanners and optical charge coupled device (CCD) scanners. Optical laser scanners generally employ a laser diode, a multifaceted polygonal mirror, focusing optics and a detector. The light from the laser diode is collimated and focused onto a multifaceted polygonal mirror. Rotation of the mirror about a central axis produces a scanning beam, which is directed to the symbology to be read. A portion of the light is reflected by the symbology back to the multifaceted polygonal mirror, which reflects the returning beam to a detector.
Typically, the scanning rate of an optical laser scanner is limited by two factors: 1) the scanning rate of the beam; and 2) the depth of field of the scanner. The scanning rate in a system having a multifaceted polygonal mirror is generally determined by the number of facets on the mirror and the available motor speed. The mirror facets must be large enough to accommodate the optical pupil diameter at the entrance to the system and, hence, the mirror structure cannot be arbitrarily small. Additionally, the mirror surface must be large enough to avoid serious vignetting whereby part of the incoming image misses the mirror toward the ends of the horizontal scan line.
With respect to CCD optical scanners, the scanning rate is limited by the speed of the CCD detector and the speed at which the optics change to the desired focal point.
The depth of field of an optical scanner is determined by factors such as optical path length and focusing optics parameters. For low volume applications and applications which require scanning of a plurality of similarly sized and shaped articles, the depth of field of the scanner is not significant since the scanner may be physically moved to focus the scanning beam. However, for applications that require high speed, high volume scanning of articles having different shapes and sizes, the depth of field of a scanner becomes critical. Although the depth of field of a scanner may be increased by mechanically repositioning a lens, this method is often too slow for high speed scanning.
Accordingly, it would be desirable to provide a means for increasing the depth of field of an imaging system without decreasing performance or increasing the cost and complexity of the system.